1. Field of the Invention
The present invention relates to both the coatings and microencapsulation arts and specifically to a sequential polymerization process for preparing a water-insoluble dispersion of core/shell particles. In one embodiment the process may be employed to produce a particulate dispersion useful in making water-based coating compositions wherein on drying the particulate dispersion serves as an opacifying agent. In another embodiment the process may be employed to microencapsulate a hydrophobic target material, such as a biocide or herbicide.
2. Brief Description of the Prior Art
Microvoid containing particles have previously been prepared by a variety of techniques.
For example, Kershaw, et al., U.S. Pat. No. 3,891,577, prepares a vesiculated polymer by converting to a solid polymer, a liquid medium containing dispersed therein particles of another polymer swollen by a liquid swellant, the liquid swellant then being at least partially removed from the dispersed polymer particle. The liquid medium may be converted to a solid by removal of solvent, e.g., from a solution of the solid polymer, or preferably, by polymerization of a monomer, comonomers, or an oligomer or a mixture of these. Optionally, a dissolved polymer may be present in the liquid to be polymerized. Solidification of the liquid in which these swollen particles are dispersed and removal of the swellant is then carried out to provide the vesiculated polymer, which may be in bulk form, as a film, or in the form of a coating applied to a substrate.
In another embodiment, Kershaw teaches that the dispersion of swollen polymer in the liquid medium may itself be dispersed in a further liquid in which it is insoluble. The further liquid is referred to as the suspending liquid. Solidification of the medium is then carried out and after separation of the granules formed from the suspending liquid, liquid swellant may be removed from the swollen polymer to provide vesiculated polymer in granular form. Alternatively, when, for example, the vesiculated granules are to be used in a coating composition with which the suspending liquid is compatible, the granules formed by solidification of the medium may be incorporated into the composition as a slurry in at least part of the suspending liquid. On applying the composition to a substrate, formation of a coating film and removal of swellant from the swollen dispersed polymer to form the vesicles within the granule then take place concurrently.
Kurth, et al., in U.S. Pat. No. 3,870,099, disclose the preparation of sequential acrylic polymers containing 0.5-2.5% of an alpha, beta=unsaturated carboxylic acid. The bulk of the acid is introduced in the early part of the polymerization.
Kowalski et al., in U.S. Pat. No. 4.427,836, disclose the production and use of water-=insoluble particulate heteropolymers made by sequential emulsion polymerization in dispersed particles of which a core of a polymeric acid is at least partially encased in a sheath polymer that is permeable to a volatile base, such as ammonia or an organic amine, adapted to cause swelling of the core by neutralization. The sheath is not permeable to permanent, non-volatile bases such as sodium hydroxide. The aqueous dispersion of the acid-containing core/sheath particles is useful in making water-based coating composition wherein it may serve as an opacifying agent when the volatile base is used to at least partially neutralize neutralize the heteropolymer, microvoids being formed in cores of the swollen particles and the film during the drying. Although the core may be made in a single stage of the sequential polymerization and the sheath may be the product of the single sequential stage following the core stage, the making of the core component may involve a plurality of steps in sequence followed by the making of the sheath which also may involve a series of sequential steps. Thus the first stage of the emulsion polymerization in the process of the Kowalski invention may be the preparation of a seed polymer containing small dispersed polymer particles insoluble in the aqueous emulsion polymerization medium. This seed polymer, which may or may not contain any acid component, provides particles of minute size which form the nuclei on which the core polymer of acid monomer, with or without nonionic comonomers, is formed. The polymer particles of this invention are prepared by aqueous emulsion polymerization, which requires a water-soluble free radical initiator, or a mixture of such an initiator with a water-soluble reducing agent to form a redox system. In a preferred embodiment a seed polymer is used along with a low level of core stage emulsifier. By carrying out the emulsion polymerization while maintaining low levels of emulsifier, the subsequent stages of polymer formation deposit the most recently formed polymer on the existing dispersed polymer particles resulting from the preceeding step or stage. If the amount of emulsifier is kept below the amount corresponding to the critical micelle concentration (CMC) for a particular monomer system, a preferred unimodal product results. While the CMC may be exceeded somewhat without the formation of an objectionable or excessive number of dispersed micelles or particles, it is preferred that the number of micelles during the various stages of polymerization be controlled so that the deposition of the subsequently formed polymer in each stage occurs upon the dispersed micelles or particles formed in the previous stages.
Kowalski et al., in related U.S. Pat. No. 4,469,825, disclose core-sheath polymer particles wherein the core monomer system requires an amine group-containing comonomer which comprises at least 5% by weight of the core monomer system.
Kowalski et al., in U.S. Ser. No. 590,082, filed Mar. 15, 1984, disclose a process for making core/sheath polymer particles in which the emulsion polymerized core system may contain either a polymerizable carboxylic acid and/or amine, giving an acid or base functional core, and in which the sheath monomer system comprises monomers having no ionizable group. A hydrophobic material such as material selected from the silicone surfactants, fluorocarbon surfactants, and hydrophobic non-vinyl polymerizable liquids, is employed in the polymerization process.
Blankenship et al., in U.S. Ser. No. 690,913, filed Jan. 11, 1985, now U.S. Pat. No. 4,594,363 disclose a process for making core-sheath polymer particles useful for opacifying coating films, comprising
(A) emulsion polymerizing a core from a core monomer system comprised of at least one ethylenically unsaturated monomer containing acid functionality;
(B) encapsulating the core with a hard sheath by emulsion polymerizing a sheath monomer system in the presence of the core, the sheath permitting penetration of fixed or permanent bases;
(C) swelling at elevated temperature the resultant core-sheath polymer particles with fixed or permanent base so as to produce a dispersion of particles which, when dried, contain a microvoid which causes opacity in compositions in which they are contained, provided that either (1) the sheath comprises at least about 1% acid functional monomer or (2) the swelling takes place in the presence of solvent.
Kowalski et al., in U.S. Pat. No. 4,468,498, discloses a process for making an aqueous dispersion of core/sheath polymers in which the core contains sufficient acid groups to render the core swellable by neutralization with a volatile base to at least twice its volume and wherein the sheath is permeable to the base.
Morehouse, Jr. et al., in U.S. Pat. No. 4,049,604, disclose aqueous dispersions of normally solid, organic polymeric particles that are prepared by (1) dispersing an oil phase containing at least one emulsion polymerizable monomer such as styrene in an aqueous phase containing a stabilizing emulsifier such as sodium dodecylbenzene sulfonate and a copolymer of a sulfo ester of an alpha, beta-ethylenically unsaturated carboxylic acid, such as 2-sulfoethyl methacrylate, and butyl acrylate and (2) subjecting the dispersion to emulsion polymerization. Microspheres having liquid centers and seamless rigid walls of the normally solid, organic polymer are prepared according to this method except that the starting oil phase also contains a nonpolymerizable, water-insoluble liquid such as hexane. The polymers of sulfo esters of alpha, beta-ethylenically unsaturated carboxylic acids serve as coalescence aids. The diameter of the resulting microspheres is inversely related to the concentration of the polymer of sulfo ester employed (operable range: 0.2 to 2.0 weight percent). Microspheres by this process have an average diameter of from about 0.5 to about 3 microns, when the amount of sulfoester employed is at the upper end of the acceptable range. Microspheres of this size are suspensions and not dispersions, they will settle out of the aqueous medium on standing.
Ugelstad, in U.S. Pat. No. 4,336,173, discloses a process for preparing an aqueous emulsion or dispersion of a partly water-soluble material and optionally further conversion of the prepared dispersion or emulsion to a polymer dispersion when the partly water-soluble material is a polymerization monomer. In the first step a dispersion of polymer particles is prepared containing one or more materials having very low solubility in water, then in a second step there is added the partly water-soluble material which diffuses into the particles from the first step, and then, if the partly water-soluble material is a polymerizable monomer, polymerization may be affected. By using a seed consisting of a polymer and essentially water-insoluble material, the seed particles will be capable of absorbing much greater amounts of monomer, it often being possible to add all the monomer in one step, and the amount of seed employed may be greatly reduced, in comparison with conventional emulsion seeded polymerization. In the conventional process, the seed particles consists of polymer molecules which are capable of absorbing only one to four times their own volume in polymerizable monomer; however, the Ugelstad seed can absorb much greater amounts of monomer. Thus the tendency to form a second mode of unseeded polymer particles during the polymerization of the monomer swollen seeds is reduced. Either a water-soluble initiator such as potassium persulfate or hydrogen peroxide or an oil-soluble initiator such as lauryl peroxide may be employed.
Ugelstad, in U.S. Pat. No. 4,113,687, disclosed a process for preparing a latex by efficiently homogenizing an aqueous mixture containing an emulsifier and a water-insoluble solvent for the monomer to be polymerized, adding monomer and, if desired, further water to the homogenized mixture and also water-soluble polymerization initiator. Instead of a water-soluble initiator, an oil soluble initiator may be used provided it has sufficient solubility diffused through the aqueous phase into the drops of water insoluble solvent and monomer.
Microencapsulation methods and the properties of the resulting microcapsulates are reviewed by T. Kondo in Surface and Colloid Science, Volume 10 (Plenum Press, New York 1978) pp. 1-41. Microencapsulation is also reviewed by R. E. Sparks in Kirk-Othmer, Encyclopedia of Chemical Technology, Volume 15 (3rd Edition) pp. 470-491. Microencapsulation of water immiscible materials, such as aqueous dispersions of pesticides and herbicides, is reviewed by Beestman et al. in U.S. Pat. Nos. 4,417,916 and 4,280,833, in which an improved microencapsulation process employing lignin sulfonate emulsifier and the reaction of polymethylene polyphenylisocyanate and a polyfunctional amine is taught. R. C. Koestler, in U.S. Pat. No. 4,360,376, teaches an interfacial polycondensation method of microencapsulating trifluralin, a pre-emergent herbicide. H. B. Scher et al., in U.S. Pat. No. 4,155,741, discloses a stable suspension-buffer system for aqueous suspensions of polyurea-microencapsulated materials, including herbicides and insecticides, which can be obtained by using aluminum hydroxide or ferric hydroxide as suspending agent, thereby preventing separation and caking in flowable microcapsule formulations.